1. Field of the Invention
The present invention relates to a blood-pressure measuring apparatus having the function of determining an augmentation index based on a cuff pulse wave occurring to a cuff worn on a living subject.
2. Related Art Statement
Augmentation index, generally known as AI, indicates, e.g., a proportion of a reflected-wave component of a pulse wave to an incident-wave component of the same, and is used to evaluate compliance of aorta. As the compliance of aorta increases, the reflected-wave component decreases and, as the compliance of aorta decreases, the reflected-wave component increases. More specifically described, if wall of aorta hardens, a reflected-wave component contained in the form of pulse wave obtained from the aorta increases. Thus, augmentation index reflects arteriosclerosis, and can be used as an index for inspecting arteriosclerosis.
As described above, augmentation index indicates a proportion of a reflected-wave component of a pulse wave to an incident-wave component of the same, but it is difficult to separate a pulse wave detected (hereinafter, referred to as a detected pulse wave) into its incident-wave component and reflected-wave component. Hence, an augmentation index may be determined as follows: First, a detected pulse wave is analyzed to identify respective peak points of an incident-wave component and a reflected-wave component of the pulse wave. Then, the augmentation index is calculated by dividing a difference between a magnitude of the pulse wave at the time of occurrence of the peak of the incident-wave component and a magnitude of the pulse wave at the time of occurrence of the peak of the reflected-wave component, by a pulse pressure of the pulse wave. In addition, the peak of the incident-wave component may be determined as an inflection point or a local maximum point between a rising point of the detected pulse wave and a peak of the same; and the peak of the reflected-wave component may be determined as the first local maximum point following the peak of the incident-wave component.
Since augmentation index is used to evaluate compliance of aorta as described above, it is a clinical practice to non-invasively detect a pulse wave from a carotid artery that is the nearest to the aorta and determine an augmentation index based on the carotid pulse wave. However, first, it needs adequate skill to wear, at an appropriate position, a carotid-pulse-wave sensor for detecting a carotid pulse wave and, second, it is needed to use or employ the carotid-pulse-wave sensor. Thus, there is a need to easily measure an augmentation index using a cuff pulse wave which is detected from a cuff worn on, e.g., an upper arm of a living subject for measuring a blood pressure of the subject.
Here, it may be possible to provide a blood-pressure measuring apparatus which measures a blood pressure using a cuff and which has the function of determining an augmentation index based on a cuff pulse wave occurring to the cuff. However, when the blood-pressure measuring apparatus having the augmentation-index determining function is used to measure a blood pressure and an augmentation index, it takes not only a pulse-wave detection time needed to keep the cuff pressure at a pulse-wave detection pressure, for detecting a cuff pulse wave to be used to determine the augmentation index, but also a blood-pressure measurement time needed to increase the cuff pressure up to a pressure higher than a systolic blood pressure of a living subject, for pressing a portion of the subject and thereby measuring the blood pressure of the subject. Thus, the subject is pressed by the cuff for an increased time and accordingly feels an increased amount of load.
It is therefore an object of the present invention to provide a blood-pressure measuring apparatus which has the function of determining an augmentation index and which can measure blood pressure and augmentation index by pressing, with a cuff, a portion of a living subject for a short time.
The above object has been achieved by the present invention according to which there is provided a blood-pressure measuring apparatus comprising a cuff which is adapted to be worn on a portion of a living subject to press the portion; an augmentation-index determining means for determining an augmentation index of the subject based on a cuff pulse wave obtained from the cuff, and a cuff-pulse-wave obtaining means for obtaining, during a pressing period in which the cuff presses the portion of the subject for measuring a blood pressure of the subject, the cuff pulse wave from the cuff so that the augmentation-index determining means determines the augmentation index based on the obtained cuff pulse wave.
According to this invention, during the pressing period in which the cuff presses the portion of the subject for measuring the blood pressure of the subject, the cuff-pulse-wave obtaining means obtains the cuff pulse wave for determining the augmentation index of the subject. Thus, the pressing period in which the cuff presses the portion of the subject for measuring the augmentation index and the blood pressure can be advantageously shortened.
According to a preferred feature of the present invention, the blood-pressure measuring apparatus further comprises a pulse-wave-detection-pressure keeping means for keeping a pressure in the cuff to a pre-determined pulse-wave detection pressure, and the cuff-pulse-wave obtaining means obtains, as the cuff pulse wave, a pressure oscillation occurring to the cuff in a state in which the pressure in the cuff is kept at the pre-determined pulse-wave detection pressure.
According to this feature, in the state in which the pressure of the cuff is kept at the pre-determined pulse-wave detection pressure by the pulse-wave-detection-pressure keeping means, the cuff-pulse-wave obtaining means obtains the cuff pulse wave. Thus, the cuff pulse wave is free from deformation caused by changing of the cuff pressure, and accordingly an accurate augmentation index can be determined based on the cuff pulse wave.
According to another feature of the present invention, the pulse-wave-detection-pressure keeping means keeps the pressure in the cuff to the pre-determined pulse-wave detection pressure, in each of an initial portion and a terminal portion of the pressing period, and the cuff-pulse-wave obtaining means obtains, as a first cuff pulse wave, a pressure oscillation occurring to the cuff in a state in which the pressure in the cuff is kept at the pre-determined pulse-wave detection pressure in the initial portion of the pressing period, and obtains, as a second cuff pulse wave, a pressure oscillation occurring to the cuff in a state in which the pressure in the cuff is kept at the pre-determined pulse-wave detection pressure in the terminal portion of the pressing period, and the augmentation-index determining means determines a first augmentation index of the subject based on the first cuff pulse wave obtained from the cuff in the initial portion of the pressing period, and determines a second augmentation index of the subject based on the second cuff pulse wave obtained from the cuff in the terminal portion of the pressing period.
According to this feature, the augmentation-index determining means determines respective augmentation indexes of the subject based on respective cuff pulse waves obtained in the initial and terminal portions of the pressing period in which the cuff presses the portion of the subject for measuring the blood pressure of the subject.
According to another feature of the present invention, the blood-pressure measuring apparatus further comprises a modified-augmentation-index determining means for determining a modified augmentation index based on the first and second augmentation indexes determined by the augmentation-index determining means.
According to this feature, the modified-augmentation-index determining means determines, based on the respective augmentation indexes determined by the augmentation-index determining means from the respective cuff pulse waves obtained in the initial and terminal portions of the pressing period in which the cuff presses the portion of the subject for measuring the blood pressure of the subject, the modified augmentation index freed from influences caused by the deformation of skin and subcutaneous tissue of the subject""s portion that contain a lot of water.
According to another feature of the present invention, the blood-pressure measuring apparatus further comprises an artery evaluating means for evaluating a degree of arteriosclerosis or a vascular endothelium of the subject based on the first and second augmentation indexes determined by the augmentation-index determining means.
For example, the artery evaluating means compares the respective cuff pulse waves, or the respective augmentation indexes, obtained in the initial and terminal portions of the pressing period, with each other, and evaluates a degree of softness of arteries of the subject based on a difference of respective amplitudes of the two cuff pulse waves, a difference of respective areas of the two cuff pulse waves, or a difference of the two augmentation indexes. In this case, as the amplitude difference of the two cuff pulse waves, the area difference of the two cuff pulse waves, or the difference of the two augmentation indexes increases, the degree of softness of arteries increases; and as the amplitude difference, the area difference or the augmentation-index difference decreases, the degree of arteriosclerosis or the degree of hardness of vascular endothelium increases.